1. Field of the Invention
This invention relates to electrical devices which are employed for supplying x-ray tubes.
2. Description of the Prior Art
An x-ray tube is provided with a cathode of the filament type which emits an electron beam in the direction of an anode or anticathode. The anode is formed of material such as tungsten or molybdenum which emits x-rays when it is bombarded by the electron beam from the cathode. In order to obtain a high-energy electron beam, the electrons are accelerated by an intense electric field produced between the cathode and the anode. To this end, the anode is brought to a positive potential of several tens of kilovolts with respect to the cathode. This potential may exceed one hundred kilovolts and attain one hundred and forty kilovolts.
Voltages of this order are provided by so-called high-voltage supply devices. As shown in FIG. 1 of the accompanying drawings, a device of this type includes a transformer 10 which is connected to voltage-doubling rectifier circuits 11. In more exact terms, the transformer 10 has a single primary winding 12 to which is applied an alternating-current voltage and a secondary circuit 13 which is connected to the voltage-doubling rectifier circuits 11. In the conventional manner, each voltage-doubling rectifier circuit 11 consists of a secondary winding 14, two diodes D1 and D2 and two capacitors C1 and C2 which are connected to each other in accordance with the diagram of FIG. 1. Each voltage-doubling rectifier circuit is connected to the following circuit so that their output voltages are added, thus making it possible to obtain a very high voltage in the last doubling circuit of the assembly.
In a typical example of construction, each voltage-doubling rectifier circuit has an output voltage of six kilovolts, so that the voltage is seventy-two kilovolts at the output of the twelfth voltage-doubling rectifier circuit.
It will be observed that, in order to obtain a potential difference of the order of 140 kilovolts between the cathode and the anode of an x-ray tube, it is only necessary to connect the cathode to a negative potential of 70 kilovolts with respect to ground and to connect the anode to a positive potential of 70 kilovolts with respect to ground. To this end, use is made of two supply devices which are identical with the device shown in FIG. 1.
It is apparent that the construction of a high-voltage device in accordance with the diagram of FIG. 1 gives rise to insulation problems which are often solved by separating conductors having very different potentials and by interposing between them an insulating medium such as oil which serves as coolant at the same time. This results in devices having large dimensions and substantial bulk.
One object of the present invention is therefore to construct a novel electrical and mechanical assembly of the different elements of a high-voltage supply device for x-ray tubes which is of small overall size and offers great ease of assembly.
Moreover, x-ray tubes are coming into increasingly widespread use in pulsed operation at repetition frequencies which are becoming steadily higher. In the circuit of FIG. 1, this means that the primary winding is supplied with an alternating-current voltage having a high frequency of the order of several tens of kilohertz. Under these novel operating conditions, the performances of the circuit of FIG. 1 are limited by the parasitic capacitances and inductances of the conductors and of the transformer windings, the values of which are difficult to determine and to compensate.
A further object of the present invention is therefore to construct a high-voltage supply device for x-ray tubes in which the relative positions of the different elements tend to minimize parasitic capacitances and inductances and contribute to a reduction in bulk of the assembly.